The present invention relates, in general, to an apparatus and method for compensating for the mass or moment of a device pivotally secured to a mounting member. More particularly, the present invention relates to an apparatus and method for attaching a range finding instrument, such as a laser range finder, to the telescope of a theodolite through the use of an improved mounting system to accurately balance and compensate for the weight of the range finding instrument throughout its vertical range of motion about the theodolite telescope pivot point.
There is a need to be able to mount a range finding instrument, such as a laser range finder, directly to the telescope of a theodolite in order to provide the capability of accurately measuring ranges as well as angles to remote targets. Since it is often necessary that the range finder be mounted to, and hence track with, the theodolite telescope, the range finder adds an additional mass and moment arm about the theodolite telescope pivot point. One common method of compensating for the additional weight has been to attach a counterbalance weight to the underside of the theodolite telescope in order to compensate for the additional moment about the pivot point due to the added weight of the range finder. This method has, among others, the disadvantages of adding extra weight to the telescope pivot point bearings as well as providing an undesirable extra weight to be carried in conjunction with the portable theodolite instrument. Moreover, due to inherent space limitations on most theodolites, the counterbalance weight must generally be heavier than the range finding instrument itself to function properly. For this reason, the use of a counterbalance is typically unsuitable, except for the most compact and lightweight range finders which might be adapted to a theodolite telescope.
Alternatively, springs have been used to balance the weight of the range finder in combination with the theodolite telescope. In this regard, two types of spring balancing systems have been utilized. A first technique utilizes a pair of compression springs housed in a pair of angularly disposed tubes that are rigidly affixed to the range finder at fixed angles thereto. The springs are engaged by a post on the theodolite housing such that rotation of the theodolite telescope causes one or the other of the springs to be compressed as the theodolite telescope is rotated about its pivot point. The force generated by the compression of the springs in these fixed tubes only approximately balances the weight of the range finder and the added weight of the range finder, when not properly balanced, makes fine adjustments to the theodolite very difficult. Moreover, due to the geometry of the fixed tubes incorporated in such system, the vertical angle through which the telescope can be rotated is typically limited to only +/-30 degrees or less from the horizontal.
The second known spring balance technique is similar to the first except that the compression springs are instead mounted in a single curved housing rather than in separate, angularly displaced tubes. The use of a curved housing for the compression springs ameliorates, somewhat, the rotational limitation problems of the first technique but not the inherent balance approximation that it provides. In fact, using a curved spring housing, the greater the angle through which the telescope is turned, the worse the balance approximation becomes. Moreover, since the springs are forced to move in a curvilinear path, considerable friction is generated between the springs and the housing which not only also interferes with the fine adjustment of the theodolite telescope but concomitantly causes undesirable wear in the mechanism.
It is, therefore, highly desirable to provide an apparatus and method for attaching a range finding instrument to the telescope of theodolite which provides accurate balance for the added weight of the range finding instrument over a wide range of vertical angles and which overcomes the aforementioned shortcomings of prior art techniques.